use the new containmentDisplayHints flags

this doesn't use any preexisting property, so won't cause regressions
due to slight behavior changes

applets/clipboard/contents/ui/ClipboardPage.qml

@@ -95,7 +95,7 @@ ColumnLayout {
                 }
             }
             PlasmaComponents3.ToolButton {
-                visible: plasmoid.containmentType !== PlasmaCore.Types.CustomEmbeddedContainment
+                visible: !(plasmoid.containmentDisplayHints & PlasmaCore.Types.ContainmentDrawsPlasmoidHeading)
 
                 icon.name: "edit-clear-history"
                 onClicked: {

applets/devicenotifier/package/contents/ui/FullRepresentation.qml

@@ -39,7 +39,7 @@ PlasmaComponents3.Page {
     Layout.minimumHeight: units.gridUnit * 12
 
     header: PlasmaExtras.BasicPlasmoidHeading {
-        visible: plasmoid.containmentType !== PlasmaCore.Types.CustomEmbeddedContainment || devicenotifier.mountedRemovables > 1
+        visible: !(plasmoid.containmentDisplayHints & PlasmaCore.Types.ContainmentDrawsPlasmoidHeading) || devicenotifier.mountedRemovables > 1
         PlasmaComponents3.ToolButton {
             id: unmountAll
             visible: devicenotifier.mountedRemovables > 1;

applets/systemtray/systemtray.cpp

@@ -48,6 +48,7 @@ SystemTray::SystemTray(QObject *parent, const QVariantList &args) :
 {
     setHasConfigurationInterface(true);
     setContainmentType(Plasma::Types::CustomEmbeddedContainment);
+    setContainmentDisplayHints(Plasma::Types::ContainmentDrawsPlasmoidHeading | Plasma::Types::ContainmentForcesSquarePlasmoids);
 }
 
 SystemTray::~SystemTray()